A number of types of EM digitizers or digitizer tablets are known in the art. In general, a digitizer tablet provides an indication of a position of a probe with respect to a surface of a planar sensor grid. One type of digitizer tablet operates by sensing a magnetic field and includes two sets of array conductors, with one set being orthogonally oriented to the other set. In this type of system the probe is driven with an AC signal to cause an oscillating magnetic field which induces signals in the array conductors. The signals in the array conductors are sensed and compared so as to determine the position of the probe in two dimensions, relative to the surface of the tablet.
Other types of digitizer tablets are also known, such as tablets that drive the array conductors and sense the field with the probe. Tablets that operate with electric fields and resistive bridges are also known. Tablets that include magnetostrictive elements are also well represented in the prior art.
The magnetic field sensing tablet is of particular interest herein. More specifically, this type of tablet measures a mutual inductance of a pair of coils. One of the pair of coils is located within the tablet and the other pair is located within the probe, also referred to herein as a stylus or pen. The pen may be physically coupled to the tablet through one or more wires, referred to as a "tethered" pen, or may be physically decoupled from the tablet as an "untethered" pen. The untethered pen embodiment is preferred in that a more natural user interface is provided.
Many tablet sensor grid arrangements consist of sets of identical, elongated loops or coils arranged along the tablet x-axis and y-axis. One conventional method of estimating pen position is to measure the signal strengths in the coils near the pen and to estimate the location of a peak signal strength along each dimension. This is equivalent to finding a zero-crossing of the signal derivative in each dimension. Since all calculations are performed on the derivative, it is convenient to refer to measurements of the derivative. These measurements are conventionally obtained by taking the differences of signals in adjacent coils from the coil array.
One problem associated with conventional digitizer tablets relates to their use in proximity to a CRT display and the coupling of external magnetic fields into the sensor grid array. The coupling of grid coil or coils to a differential amplifier through multiplexer circuitry is a known technique for reducing common mode noise.
Another problem relates to the finite extent of the sensor grid coils that results in erroneous pen position estimations for pen positions at or approaching the edges of an active area of the sensor grid array.
The following chronologically ordered U.S. patents are representative of a significant number U.S. patents that have been issued in this technical area.
In U.S. Pat. No. 4,185,165, issued Jan. 22, 1980, entitled "Low Noise System and Method for Sequentially Sensing Induced Signals in Digitizer Grid Conductors" to Fencl there is described a digitizing system having a pen with a coil disposed around a nose piece. A grid of conductors includes a group of parallel spaced conductors oriented in X and Y directions. A 96 KHz alternating current voltage is induced in the X and Y conductors by the pen coil and a differential amplifier is connected to the top and the bottom of a single coil through multiplexers.
In U.S. Pat. No. 4,552,991, issued Nov. 12, 1985, entitled "Absolute Position Coordinate Determining Device Employing a Single Phase Difference Measurement to Determine Cursor Position" to Hulls there is described a two dimensional digitizing system that includes at least two conducting grid systems. Each grid system has a number of windings with a first pitch distance and an equal number of windings with a second pitch distance that differs from the first pitch distance by a small increment. This technique is said to enable both coarse and fine position measurements.
U.S. Pat. No. 4,644,102, issued Feb. 17, 1987, entitled "Digitizing Tablet System" to Blesser et al. there is described a digitizing tablet that includes a differential amplifier that receives inputs from multiplexers and provides output signals to a processor. Two multiplexers and a differential amplifier are said to provide an immediate comparison between the signals on adjacent conductive elements. The resulting signal is said to be stripped of common mode noise and unwanted common mode signal, thus increasing the signal-to-noise ratio.
In U.S. Pat. No. 4,736,073 issued Apr. 5, 1988, entitled "Noise-Canceling System for a Digitizing Tablet" to Abernethy there is described a digitizing tablet having a signal source that drives the coil in a probe with an oscillating current so that the probe generates a varying magnetic field. The field induces electromotive force in a first set and a second set of array conductors oriented at right angles to one another. Return circuitry is provided so that a differential amplifier can amplify the signals induced in the conductors. A return circuit is provided in the vicinity of an edge of the tablet for dealing with inaccuracies due to edge effects.
In U.S. Pat. No. 4,859,814, issued Aug. 22, 1989, entitled "Noise Cancelling in Digitizing System and Method" to Sciacero et al. there is described a digitizing system said to detect and cancel ambient noise from detected grid carrier signals before processing the grid carrier signals to compute the coordinates of a pointing device. A differential noise reduction technique is disclosed. When a grid conductor is selected for sensing, another grid conductor that is located a certain number of grid conductors away is also selected as a reference conductor of a pair of differentially sensed grid conductors.
What is not taught by this prior art, and what is thus an object of the invention to provide, is an improved digitizer tablet that employs arrays of differentially connected coil pairs for achieving a significant rejection of common mode noise signals, while reducing multiplexer complexity.
A further object of the invention is to improve accuracy near the boundaries of a digitizer tablet by compensating for the finite extent of the sensor grid coils.